Mycobacterium avium is generally nonpathogenic in healthy individuals but causes invasive, disseminated infection in AIDS patients. We have found that three critical variables affect the outcome of the interaction of M. avium with the host's immune system: (i) the strain of M. avium; (ii) the presence of concurrent HIV-I infection of the macrophage; and (iii) the immune status of the host with regard to M. avium and M. tuberculosis. To define the role of these variables in the interaction of M. avium with the host we propose the following work: We will use an RNA subtractive hybridization procedure to identify M. avium gene(s) that are specifically expressed during growth in host cells and not during growth in routine media. These experiments will be performed using epidemiologically related clinical and environmental M. avium isolates that are genotypically indistinguishable. The genes identified will be cloned, expressed in M. smegmatis, and examined for phenotypic effects using assays for virulence in PBMCs and Macrophage. We will characterize M. avium isolates genotypically and phenotypically to correlate genotypes (particularly the presence of the gene(s) identified above) with virulence for PBMCs and Macrophage in vitro and to determine if the virulent strains of M. avium are restricted to a limited set of genetic lineages. We will examine the function of different leukocyte populations and of different cytokines in mediating resistance of human Macrophage to infection in vitro with virulent and avirulent M. avium strains. We will also determine how cytokine production varies with HIV-1 coinfection of Macrophage and with the presence of host immunity to mycobacteria. We will determine if the virulence of M. avium is altered in vitro by exposure to host cytokines and passage of strains in Macrophage. We will also culture environmental sources to determine the distribution of virulent strains of M. avium within the environment and the reservoir of- pathogenic strains. These studies will allow us (i) to identify and clone M. avium genes associated with virulence; (ii) to define the role of cytokines and leukocyte populations in the host defense against mycobacteria; and (iii) to determine if specific host factors induce virulence in M. avium strains. Cumulatively, information from these studies will contribute to the development of rational therapeutic modalities for mycobacterial infections in the immunocompromised host.